To determine whether treatment of schistosomiasis has an effect on the course of human immunodeficiency virus type 1 (HIV-1) infection, individuals with schistosomiasis and with or without HIV-1 infection were randomized to receive praziquantel treatment at inclusion or after a delay of 3 months; 287 participants were included in the study, and 227 (79%) were followed up. Among the 130 participants who were coinfected, those who received early treatment (n=64) had a significantly lower increase in plasma HIV-1 RNA load than did those who received delayed treatment (n=66) (P<.05); this difference was associated with no change in plasma HIV-1 RNA load in the early intervention group (P=.99) and an increase in plasma HIV-1 RNA load in the delayed intervention group (P<.01). Among the 227 participants who were followed up, those who received early treatment (n=105) had an increase in CD4 cell count, whereas those who received delayed treatment (n=122) did not (P<.05); this effect did not differ between participants when stratified by HIV-1 infection status (P=.17). The present study suggests that treatment of schistosomiasis can reduce the rate of viral replication and increase CD4 cell count in the coinfected host.